The heart of a computer's long term memory is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider toward the surface of the disk and when the disk rotates, air adjacent to the surface of the disk moves along with the disk. The slider flies on this moving air at a very low elevation (fly height) over the surface of the disk. This fly height is on the order of Angstroms. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic transitions to and reading magnetic transitions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. This sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is biased parallel to the ABS, but is free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos Θ, where Θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetic moments of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP1) with a layer of antiferromagnetic material such as PtMn. While an antiferromagnetic (AFM) material such as PtMn does not in and of itself have a magnetic moment, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.
The free layer is biased by a hard magnets constructed as layers of high magnetic coercivity material formed at each lateral side of the sensor. Magnetostatic coupling between each hard bias layer and the free layer biases the magnetic moment of the free layer in a direction parallel to the ABS as desired, but allows the magnetic moment of the free layer to rotate in the presence of a field that is sufficiently strong to overcome the biasing. As sensor are made ever smaller, the free layers become less stable and more difficult to bias. The hard bias structures that have been used can not provide sufficient magnetic moment to bias and stabilize the free layers of future generation sensors. This problem is made worse when combined with partial mill designs which are needed to reduce gap height. The necessity for such partial mill designs and the challenges that they present to free layer biasing are described as follows.
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
As those skilled in the art will appreciate, the distance between the shields (gap thickness) determines the bit length for the read sensor. With the ever increasing pressure to increase data capacity and data rate, engineers are constantly under pressure to decrease the bit length (and therefore gap thickness) of read sensors. One way to decrease this gap thickness is by a partial mill process. Sensors have traditionally been constructed by depositing sensor layers as full film layers onto the first gap layer. A photoresist mask is then formed over the area that is to be the sensor and a material removal process such as ion milling is performed to remove material from the areas not covered by the mask. This material removal process has traditionally been performed down into the first gap layer, removing a portion of the first gap material at either side of the sensor.
Since this material removal process removes a portion of the first gap layer, it has been necessary to deposit a thick first gap layer in order prevent electrical shorts through the gap to the first shield. Such a short would be a catastrophic event that would render the sensor unusable. In these prior art heads, hard bias layers, constructed of a hard magnetic material such as CoPtCr have then been deposited over this etched out portion of the first gap layer at either side of the sensor to provide magnetic biasing to bias the magnetic moment of the free layer in the desired direction parallel with the ABS.
As discussed above, the removal of a portion of the first gap during formation of the sensor requires a larger overall gap thickness to prevent shorting. One way to overcome this is to use a partial mill process in which the material removal process (ie. ion milling) used to construct the sensor is terminated before all of the sensor material has been removed, such as when the material removal process has reached a point around the AFM layer (usually PtMn) near the bottom of the sensor. By stopping the milling process within the sensor layers, such as at the AFM layer no gap material is removed. This allows a much thinner gap to be deposited. The bias layers are then deposited on top of the remaining sensor layer rather than on the gap layer.
A problem that arises from such partial milling is that the bias layer properties are different when deposited over the AFM layer or other sensor layer than they are when deposited over the gap layer. The gap layer, usually Al2O3 is amorphous. It therefore has no crystalline structure to impart to the seed layer or to the hard bias material when the material is deposited onto the gap. Therefore, a hard bias structure deposited over the amorphous gap layer can exhibit a desired epitaxial growth that provides desired high retentive moment and high coercivity needed for free layer biasing.
However, the AFM layer, such as for example PtMn, as well as other sensor layers are not amorphous and exhibit their own grain structures. When the hard bias layers are deposited over the AFM layer, the grain structure of the underlying AFM layer carries through to the seed layer and hard bias layers. This grain structure being undesirable for optimal hard bias properties results in degraded biasing properties. This leads to free layer instability and associated signal noise. For example, depositing a CoPtCr hard magnetic material with a Cr seed layer over a PtMn substrate results in a CoPtCr hard bias layer with a coercivity of only around 600 Oe. This is much lower than the roughly 1400 Oe coercivity obtained when the same hard bias layer and seed are deposited on a glass (amorphous) substrate.
Another challenge to free layer biasing comes from the necessity to apply a relatively thick seed layer under the hard bias layer. In order to achieve the desired magnetic properties in the hard bias layers, a seed layer must first be deposited. This seed layer covers the sides of the sensor, and defines the separation between hard bias layer and the free layer. Since the magnetostatic coupling provided by the hard bias layers drops off nearly exponentially with increased distance between the hard bias layer and the free layer, any increase in seed layer thickness dramatically affects free layer efficiency.
Therefore there is a strong felt need for a hard bias structure that can provide increased biasing for a magnetoresistive sensor. Such a bias structure must provide increased biasing and improved magnetostatic coupling between the bias layers and the free layer. Such a bias structure should also capable of being deposited over a crystalline structure such as an AFM material or other sensor material while still exhibiting the necessary hard magnetic properties needed for effective free layer biasing.